Not applicable.
Not applicable.
The process of the present invention relates to treating waste water by reverse osmosis. More specifically, this process involves treating waste water that contains hydrofluoric acid (HF) and mixed acid etchant (MAE) waste.
Currently, waste water containing HF and MAE waste is generated by silicon wafer manufacturers. MAE waste contains mixtures of acids including hydrofluoric acid and nitric acid (NHO3). It also may contain acetic acid (CH3CO2H). Still further, the MAE waste contains dissolved silica (SiO2). Dissolved silica may also be found in the HF stream that is part of the waste water. Conventionally, this waste water is treated with calcium hydroxide (Ca(OH)2, which is lime) or calcium chloride (CaCl2) to precipitate fluoride. The disadvantage with this process is that it is difficult to produce clean and reusable water, and instead, much of the waste water must be disposed. Another disadvantage with this process is that when CaCl2 is used, it is especially difficult to reclaim the water due to the presence of a high concentration of chloride. Another disadvantage is that lime or calcium chloride treatment does not remove nitrates. Another method for treating this waste is by biological denitrification so as to remove nitrates from the waste water. However, such a process requires expensive capital costs. Still further, both of these conventional processes are not able to concentrate the waste being removed, and therefore, these processes require large vessels for processing the waste water.
It is an object of the present invention to provide a process for treating waste water containing hydrofluoric acid, mixed acid etchant waste, and dissolved silica that is able to produce clean, reusable water.
Another object of the present invention is to provide a waste water treatment system that results in lower capital costs compared to conventional systems because the waste is concentrated thus requiring smaller treatment vessels.
According to the present invention, the foregoing and other objects are achieved by a process for treating waste water containing hydrofluoric acid, mixed acid etchant waste, dissolved silica, and solid particles and a system for performing this process. This process involves adjusting the pH of the waste water to about 7 or above, filtering the waste water to remove solid particles, adding an anti-scalant to the waste water, and feeding the waste water through a reverse osmosis membrane. The reverse osmosis membrane separates the waste water into a permeate stream, which product, and a reject stream.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.